This invention relates to the simultaneous incorporation of silicon and aluminum into nickel alloy surfaces that have been enriched in platinum, to produce a uniquely protective coating with significantly improved resistance to hot corrosion and oxidation than that which can be achieved by additions of either silicon or platinum alone. The coating comprises platinum and nickel aluminide phases that are relatively free of substrate elements, particularly refractory metals, which hinder performance, said elements being concentrated within silicide compounds which contribute to the overall corrosion resistance of the coating layer.
During operation, components in the hot section (or power turbine section) of a gas turbine are exposed to temperatures that can reach 1200.degree. C. These components are typically made of nickel and cobalt base alloys specially fabricated for high temperature use. Even so, upon exposure to service at such high temperatures, these heat resistant materials begin to revert to their natural form, metal oxides and/or sulfides. Nickel and cobalt oxides are not tightly adherent. During thermal cycling, they crack and spall off the surface exposing more substrate to the environment. In this manner, oxidation roughens and eventually consumes unprotected parts made of these alloys (see FIG. 1).
Sodium, chlorine and sulfur in the operating environment speed degradation. Above about 540.degree. C., sodium reacts with sulfur-containing compounds to form molten sulfates which condense on the metal parts, dissolving the loosely adherent films of nickel and cobalt oxide and attacking the substrate (see FIG. 2).
The chemistry of high-temperature superalloys was initially optimized for high-temperature strength. Refractory elements such as molybdenum, tungsten and vanadium were added to enhance high-temperature strength of nickel-base alloys. However, it became apparent with time that these same refractory elements, though beneficial for alloy strength, seriously reduced high-temperature corrosion resistance. It became necessary to modify alloy chemistries for service in corrosive environments by increasing levels of chromium, which has a beneficial effect on alloy corrosion resistance. Chromium, however, reduces the high temperature strength of nickel-base superalloys.
One means to enhance oxidation and hot corrosion resistance of nickel and cobalt superalloys, widely known in the art and practiced in gas turbine engines, is to alloy aluminum into the surface of the parts. Aluminum forms stable intermetallic compounds with both nickel and cobalt. When the concentration of aluminum in these phases is sufficiently high, the oxide scale which forms at high temperature is no longer a loosely adherent base metal oxide, but a tough, tightly adherent, protective layer of alumina (Al.sub.2 O.sub.3) (see FIG. 3).
Wachtell et al., U.S. Pat. No. 3,257,230, and Boone et al., U.S. Pat. No. 3,544,348, are among those who have described methods of forming these protective layers of intermetallic aluminide from an aluminum vapor in a process known as "pack" aluminizing. Aluminum or aluminum alloy powders are mixed with inert powder (usually alumina) and halide compounds known as activators. When heated to sufficiently high temperatures (650.degree. C. or more), the halides react with the aluminum to form gaseous aluminum halides. These vapors condense on the metal surface, where they are reduced to elemental aluminum. These aluminum atoms diffuse into the substrate to form protective intermetallic aluminide phases--NiAl and Ni.sub.2 Al.sub.3 on nickel alloy substrates and CoAl and Co.sub.2 Al.sub.5 on cobalt alloys.
Joseph, U.S. Pat. No. 3,102,044 describes, how a protective layer of intermetallic aluminides may be produced from liquid phase reactions of a metal-filled coating on the surface of a part. In this process, known as slurry aluminizing, a layer of aluminum metal is deposited on the hardware, then the part is heated in a protective atmosphere. When the temperature exceeds the melting temperature of aluminum (660.degree. C.), the aluminum metal on the surface melts and reacts with the substrate. NiAl forms directly, avoiding formation of higher aluminum content intermetallics.
One commercial slurry aluminizing coating method used in the aircraft industry, specifies that aluminum be deposited on the surface before diffusion by means of thermal spray or application of a metal-filled slurry or paint. One slurry used is an aluminum-filled chromate/phosphate slurry such as that described in Allen, U.S. Pat. No. 3,248,251. This slurry consists of aluminum powders in an acidic water-based solution of chromates and phosphates. The slurry can be applied by brush or conventional spray methods. When heated at a temperature of about 260.degree. C. to 540.degree. C. (500.degree. F. to 1000.degree. F.) , the binder transforms to a glassy solid which bonds the metal powder particles to one another and the substrate.
It has been found that when a slurry coated superalloy part is heated to temperatures of about 980.degree. C. (1800.degree. F.), the aluminum powder melts and diffuses into the part to produce a protective aluminide, that is, NiAl on a nickel alloy and CoAl on a cobalt alloy. Because the ceramic binder is stable at the processing temperatures, the aluminum powder is firmly held against the substrate as diffusion proceeds. Deadmore et al., U.S. Pat. No. 4,310,574, describes a means to enhance hot corrosion resistance of an aluminide by simultaneously incorporating silicon into the surface during aluminization. In this patent, a silicon-filled organic slurry is sprayed onto a part which is then placed into a pack mixture of aluminum and activators. During heating, aluminum condensing on the surface carries silicon with it as it diffuses into the substrate. It was shown that the resulting silicon-enriched aluminide was more resistant to oxidation at 1093.degree. C. (2000.degree. F.) than were aluminides without silicon.
Another means for adding silicon to an aluminide coating, which predates the Deadmore '574 patent, is to simultaneously melt and alloy aluminum and silicon into the surface. An aluminum and silicon-filled slurry available commercially under the tradename SermaLoy.RTM. J (Sermatech International, Limerick, Pa., U.S.A.), has been used for many years to repair imperfections and touch up parts coated with pack aluminides and MCrAlY overlay coatings. In SermaLoy.RTM. J slurry, aluminum and silicon powders are dispersed in a chromate/phosphate binder of the type described in the Allen '251 patent.
As supplied for use, the SermaLoy.RTM. J slurry coating composition comprises silicon and aluminum elemental metallic powders in an acidic water solution of inorganic salts as a binder. About 15% by weight of the total metallic powder content of the slurry is silicon powder. However, the overall composition of the slurry in approximate weight percentages is:
Al powder--35% PA1 Si powder--6% PA1 Water--47% PA1 Binder salts (dissolved in the water)--12%
A preferred mode of preparation of the composition is to premix the metallic powder constituents and make the binder solution separately, then mix the powder into the solution. Other ways of preparing the composition can readily be devised.
This binder is selected to cure to a solid matrix which holds the metal pigments in contact with the metal surface during heating to the diffusion temperature. It also is selected to be fugitive during diffusion to yield residues that are only loosely adherent to the surface after diffusion has been completed.
When a nickel alloy coated with SermaLoy.RTM. J slurry is heated to 870.degree. C. (1600.degree. F.), aluminum powder in the slurry melts, silicon powder dissolves into this molten aluminum and both species diffuse into and alloy with the substrate.
The intermetallic phases that result are formed by inward diffusion of these metals. Diffusion is biased by the different affinities of the diffusing species for elements in the substrate. On nickel alloys, aluminum reacts with nickel while silicon segregates to chromium and other refractory elements. The result is a composite coating of beta-phase nickel aluminide (NiAl) and chromium silicides (Cr.sub.x Si.sub.y). The unique layered structure of this composite coating on a Waspaloy.RTM. nickel superalloy substrate is shown in FIG. 4. Layering of nickel, chromium, silicon, aluminum and cobalt phases within this structure is shown in the electron microprobe maps in FIGS. 5a-e.
Engine experience and laboratory testing affirm that this aluminide-silicide coating is more resistant to sulfidation and hot corrosion than aluminides not modified with silicon in this manner. Silicides in these slurry aluminides are especially resistant to attack by molten sulfates, so the layers (in FIG. 4) act as barriers to hot corrosion.
However, it has been found that the corrosion resistance of silicon-modified slurry aluminide coatings depends upon the chromium content of the underlying substrate metal. In laboratory burner rig tests, the performance of a silicon-modified coating on IN738, which contains about 16% chromium, is significantly better than that of the same coating on IN100, a nickel alloy containing about 10% chromium. The hot corrosion life of a SermaLoy.RTM. J coating was 300-350 hours/mil (12-14 hrs/.mu.m) when tested on IN738. The corrosion life of the coating was only 150-200 hrs/nil (6-8 hrs/.mu.m) on IN100.
Bungardt et al. (U.S. Pat. Nos. 3,677,789 and 3,819,338) show that hot corrosion and oxidation resistance of diffused aluminides may be enhanced by incorporating metals of the platinum group. At least 3 to 7 .mu.m of platinum is electroplated onto a nickel surface. The platinum layer is diffused into the substrate by pack aluminization at temperatures of about 1100.degree. C. to form a protective diffusion layer on the surface. When the platinum-coated surface is aluminized in a pack, a portion of intermetallic aluminides which form are platinum-aluminides (PtAl and PtAl.sub.2) rather than nickel-aluminides. The aluminum oxide scale that forms on such a mixture of platinum and nickel aluminides is tougher and more adherent than the scale that forms on nickel aluminides alone.
Others in addition to Bungardt have capitalized upon the performance improvement expected due to replacing some portion of the nickel aluminide in a high temperature coating with platinum aluminides. Stueber et al. (U.S. Pat. Nos. 3,999,956 and 4,070,507), for example, shows that the benefits of platinum can be augmented by incorporating rhodium into the aluminide as well. Panzera et al. (U.S. Pat. No. 3,979,273) describes how these benefits might be realized by alloying thinner deposits of platinum with active elements like Y, Zr or Hf. Shankar et al. (U.S. Pat. No. 4,526,814) describe protective aluminides formed by diffusing chromium and platinum into nickel surfaces before aluminizing. The chromium improves the corrosion resistance of the nickel aluminide phase, thereby substantially improving the overall performance of the platinum-modified aluminide.
Creech et al. (U.S. Pat. No. 5,057,196) describe a method for improving mechanical properties of platinum modified aluminide coatings. In their method, a platinum-silicon alloy powder is electrophoretically deposited on the surface, then heated to a sufficient temperature to melt the alloy powder and initiate diffusion of the platinum and silicon into the nickel substrate. Subsequently, aluminum-chromium powder is diffused through this platinum-silicon-nickel alloy layer to produce an aluminide coating. The patent indicates that incorporating silicon into the coating by co-diffusing with platinum improves ductility over such a coating without silicon.
Despite advancements and modifications to diffusion aluminide coating processes, the high-temperature corrosion performance of current coatings of this type is generally affected by substrate alloy chemistry. A diffusion aluminide coating applied on an alloy substrate optimized for high-temperature corrosion resistance (that is, high chromium content) will perform significantly better than the same coating applied on an alloy substrate with poor high-temperature corrosion resistance (that is, low chromium contact). This inherent limitation of current practice restrains the utilization of stronger or less expensive alloys (with correspondingly lower chromium contents) from applications where high-temperature corrosion is prevalent, such as marine gas turbines and offshore power generation.
Background technical articles of interest are the following. The benefits of silicon-based coatings have been described by F. Fitzer and J. Schlicting in their paper "Coatings Containing Chromium, Aluminum and Silicon" for National Association of Corrosion Engineers held Mar. 2-6, 1981 in San Diego, Calif., and published as pages 604-614 of "High Temperature Corrosion", (Ed. Robert A. Rapp). Details of testing of rotor blade materials and coatings have been published by the American Society of Mechanical Engineers (ASME) in a paper by R. N. Davis and C. E. Grinell entitled "Engine Experience of Turbine Materials and Coatings (1982). Also see "Protective Coatings For High Temperature Alloys State of Technology", by G. William Goward, from "Proceedings of the Electrochemical Society, Vol 77-1", "Strengthening Mechanisms in Nickel-Base Superalloys", by R. F. Decker, presented at the Steel Strengthening Mechanisms Symposium in Zurich, Switzerland on May 5th and 6th, 1969, and "High Temperature High Strength Nickel Base Alloys", a publication of International Nickel, Inc. of SaddleBrook, N.J. All of these publications are incorporated herein by reference.